The term inkjet refers to a printer system that ejects a drop of ink on demand through an opening in the head of a printer cartridge. The ink in an inkjet cartridge is dispensed from the large cartridge reservoir into a much smaller pressurized reservoir where the ink is separated into individual channels. The ink funnels through the channel to the opening in a nozzle plate. Behind this opening is a tiny heater. When the heater reaches a certain temperature, the ink in contact with the heater vaporizes and is ejected out through the nozzle opening. The ejected ink forms a droplet that upon hitting a substrate such as paper, becomes a dot. When many ink droplets or dots are combined in any given pattern, they can form a letter, line, character or symbol. The ejection of the ink drop gives rise to the term inkjet.
To date, most channels through which the ink is distributed to the heaters are defined by dry film photoresist (see FIG. 1), Dry films are organic films that are laminated to a substrate using heat and pressure. The film is then defined using a photo process similar to that of printed circuit boards.
The dry film materials were originally developed for printed circuit boards and are now becoming obsolete. For example, the lithographic properties of dry films are limited to approximately 2-4 mil lines/spaces in 20 mil thick films. Newer inkjet print heads will require 8 .mu.lines/spaces in 30+.mu. thick films. Also, the inks used in inkjet cartridges can have a pH as high as 9. The materials used in dry film resists are subject to attack at this high pH. If the channel walls deteriorate, the pressure of the ejected ink drop changes causing drop distortion and a decline in print quality. Worse case, the deterioration could become so severe that the channel wall breaks down causing the reservoir to collapse and the adhesion of the thermally bonded nozzle plate to break down. This would be catastrophic to the print head.
Dry films have also been a cause for environmental concern. It is known that in the past, many of the dry films that meet inkjet fabrication specifications have been manufactured using chlorinated solvents for example. It is also known that the processing of dry films generates large quantities of waste in the form of trim. As a result, the companies that provide these materials are phasing out existing product lines and attempting to replace them with more environmentally friendly versions. The newer dry films are most commonly developed with aqueous base. As a result many recently evaluated dry films did not stand up to the high pH inks and could not attain the smaller dimensions required by newer print head designs.
The spin-on epoxy based resist described herein can be formulated in `safe` solvents reducing possible environmental impact. Since it is a spin-on material, there is potentially less waste because less material is used. For example a typical six inch wafer requires &lt;8 cc of liquid resist whereas dry films generate trim waste of unused material around the substrate as well as the disposal of the top and bottom support sheets.
The material described herein was developed to replace an existing product while extending the material properties, such as greater resolution, higher aspect ratios and adhesion to metal surfaces such as gold or gold/tantalum, thereby extending the materials application to present and projected product requirements. This material provides a permanently define, high pH ink resistant barrier that can contribute to controlled drop size in pressurized inkjet heads without loss of bond strength between the material and the gold or gold/tantalum coated nozzle plate.
It is an object of the present invention to provide a material that is an epoxy based photoresist in an environmentally acceptable solvent system that can replace present dry film resists.
It is another object of the present invention to provide a material that yields high aspect ratio lithographic images that when cured can become part of a device such as an inkjet print head or a micromachining sub structures.
It is another object of the present invention to provide a material developed to replace an existing dry film resist that lacked the resolution or extendibility required for possible future inkjet head designs.